Cement is a fine, gray powder consisting of alumina, lime, silica, and iron oxide which sets to a hard material after mixture with water. Cement, along with sand and stone aggregate, make up concrete, the most widely used building material in the world. Steel reinforcing bars (rebars) are commonly added to the interior of concrete for additional strength.
There are many reasons for the popularity of concrete. It is relatively inexpensive, capable of taking on the shape of a mold, exceptionally high in compression strength, and very durable when not exposed to repeated freeze-thaw cycles. However, whether or not reinforced, concrete is not without its drawbacks. One major drawback of concrete is that it is relatively low in tensile strength. In other words, it has little ability to bend, has little impact resistance, and is brittle. A second major drawback is that its durability is significantly reduced when exposed to repeated freeze-thaw cycles in the presence of water. Concrete is relatively porous and water is able to permeate the material. Freezing and thawing, with the accompanying expansion and contraction of the water, forms cracks in the concrete. Furthermore, if salt is also present in the environment, it dissolves in the water and permeates into the concrete where it induces corrosion of any rebars present.
Various techniques have been suggested for addressing these drawbacks. The addition of fibers to concrete has improved its tensile strength, but has decreased its compression strength. Coating the exterior surface of the concrete reduces water permeation, but is a time-consuming additional step and has little, if any, effect on the strength of the concrete. Accordingly, a demand still exists for a concrete with greater tensile strength, greater durability, and comparable compression strength.